1. Field of the Invention
The present invention relates to a multimode access control method performed by a communication terminal capable of communicating with a plurality of cellular systems and a multimode mobile station using the multimode access control method.
2. Description of the Related Art
To date, a plurality of cellular systems, such as CDMA 2000 (Code Division Multiple Access 2000), EV-DO (Evolution-Data Only), W-CDMA (a European version of Wideband CDMA), etc., have been in practical use. Also, it is assumed that the same cellular system will be operated at different frequency bands, for example, an 800-MHz band, a 1.7-GHz band, a 2-GHz band, etc., in the future. Also, multimode mobile communication terminals capable of using such a plurality of cellular systems or a plurality of frequency bands have been developed.
When the power is turned on, a multimode mobile communication terminal performs the acquisition operation of the control signal transmitted by a base station of each system (including the cases of having different frequency bands within the same system) in the order based on the information of a preferred list which indicates available systems stored in that terminal in advance. When the synchronization is obtained, the terminal enters an idle state. Also, when the control signal of the system which the terminal has acquired so far is lost (system lost), or when maintaining the idle operation becomes difficult because of the deterioration of the radio wave environment or the like, the terminal performs the acquisition operation on the basis of the order based on the information of the preferred list which indicates available systems and enters an idle state of the system that has been re-acquired.
Techniques for changing the order of acquisition among a plurality of systems in accordance with the reception quality (RSSI and Ec/Io) at an idle time are known. For example, the following technique has been disclosed in Japanese Laid-Open (Kokai) Patent Application Publication No. 2003-153325. In the technique, when two systems, A and B have been registered in this order on a preferred list which indicates available systems and a system lost occurs in an idle state of system A, if the reception quality of system A is better than a certain condition, the probability of re-acquiring system A is high and the existence of system B is not sure. Thus, the acquisition operation is tried again in the order of A→B. On the other hand, if the reception quality of system A is worse than the certain condition, the probability of re-acquiring system A is low. Thus the acquisition operation of system A is not performed and the acquisition operation of system B is performed from the beginning in order to shorten re-acquisition time and to reduce power consumption.
However, the method disclosed in the above-described Japanese Laid-Open Patent Application Publication cannot be applied to the case where a current cellular system which the mobile station has acquired is different from a cellular system which the mobile station tries to access. A description will be given on this with reference to a known access sequence shown in FIG. 3. It is assumed that the multimode mobile communication terminal performing the access sequence shown in FIG. 3 can be used with three systems, A, B and C.
First, when an access operation is performed (S102) in an idle state with system C (S100), the acquisition operation of system A is performed (S104). Here, if system A is acquired, the communication with system A is started (S106). Whereas if it is not acquired, the processing proceeds to the acquisition operation of system B (S108). Here, if system B is acquired, the communication with system B is started (S110). Whereas if it is not acquired, the processing proceeds to the acquisition operation of system C (S112). Here, if system C is acquired, the communication with system C is started (S114). Whereas if it is not acquired, an access failure occurs and the processing proceeds to an idle state (S116).
In a multimode mobile communication terminal performing such an operation, for example, it becomes a waste of time and a waste of power consumption to perform the acquisition operation of system A in step S104 in an area out of the service range of system A. In this case, it is desirable to proceed to the acquisition operation of system B or system C without performing the acquisition operation of system A. However, since the reception quality of system C, which has been acquired, has no correlation with the acquisition easiness of system A or system B in general, a problem arises that advantages of shortening re-acquisition time and reducing power consumption cannot be obtained even if the acquisition order is changed in accordance with the reception quality at an idle time as in the known technique.